Chlamydia trachomatis type III secretion: evidence for a functional apparatus during early-cycle development

Area of Science:

• Microbiology
• Cell Biology
• Bacterial Pathogenesis

Background:

• Chlamydia trachomatis establishes an intracellular niche within a vacuole called an inclusion.
• It utilizes a type III secretion system (T3SS) to deploy effector proteins, including inclusion membrane proteins (Incs).
• Early secreted Incs appear before detectable T3SS gene expression, creating a paradox.

Purpose of the Study:

• To investigate the presence and function of the T3SS apparatus on Chlamydia trachomatis elementary bodies (EBs) during the early stages of infection.
• To resolve the temporal paradox between early Inc secretion and later T3SS gene expression.

Main Methods:

• Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis of purified EB extracts.
• Immunoblot analysis using antibodies against the T3SS component CdsJ.
• Indirect immunofluorescence of infected cells.
• Heterologous expression of IncC in Yersinia pseudotuberculosis.

Main Results:

• The T3SS apparatus was detected on purified EBs.
• CdsJ, a core T3SS component, was identified in both EB and reticulate body (RB) extracts.
• CdsJ was detected before its corresponding mRNA, and IncC was secreted via a heterologous T3SS.
• Early Incs are secreted via T3SS pores present on EBs.

Conclusions:

• A model is proposed where EBs carry pre-formed T3SS pores for early effector secretion.
• Mid-cycle T3SS gene expression replenishes the apparatus on RBs and provides pores for nascent EBs.
• This mechanism explains the early secretion of effector proteins by Chlamydia trachomatis.